Re: [Full-Disclosure] Small (but useful) utility

From: Andrew Farmer (andfarm_at_teknovis.com)
Date: 08/05/04

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    To: "M. Mohr" <m.mohr@laposte.net>
Date: Thu, 5 Aug 2004 13:29:07 -0700

    On 5 Aug 2004, at 01:07, M.. Mohr wrote:
    > When I couldn't find a decent file wiping utility on my own
    > machine, I decided to write one. Yes, I did search the net
    > and came up with a few... but they seem to be poorly written
    > and overly complicated.

    shred(1)? It's in coreutils, for heavens' sake!

    I'll take a look at this for educational purposes.

    > You might want to name this program something inconspicuous
    > (like 'index') and place it in /sbin, then add something like
    > the following to rc.local:
    >
    > sleep 300 && screen -d -m find /home/foo -type f | xargs
    > /sbin/index
    >
    > Just make sure to resume the screen as root within 5 minutes
    > of system boot time :)

    Er... kill the sleep within 5 minutes, maybe. And hope your
    power never goes out unexpectedly (eep!) Not recommended if you
    value your data.

    As to the code, though...

    1. writeout() doesn't deal with the "tail" of a file. Round the
        file size up to the nearest multiple of the file system's
        blocksize - probably 8K or so.

    2. rand() isn't random enough for this. Use

    3. Writing random bytes LAST is useless - the data can be read
        off the disk and "subtracted" from the signal to yield the
        previous level of data. If you're serious about wiping data,
        the best way to go with modern drives is to write several
        layers of random data.

        for x in 1 2 3; do dd if=/dev/urandom of=file; done

    4. I don't know much about the buffering issues involved here,
        but you could at least run an fsync() after each pass to try
        to get the OS to write out each run of data.

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